1. Field of the Invention
The present invention relates to a method of manufacturing a photomask which is used in lithographic process during fabrication of a semiconductor device or the like.
2. Description of the Prior Art
A photomask for use in lithographic process in general includes a predetermined transfer pattern which consists of a portion transparent to illuminated light and a portion opaque to the illuminated light. The transfer pattern formed on the photomask is projected by a lens system onto a target substrate which includes a photosensitive material layer, and eventually transferred onto the photosensitive material layer.
FIG. 13 is a cross sectional view of a conventional photomask. On a transparent substrate 31 made of glass or other suitable material, light shielding members 32 made of Cr, MoSi and the like are formed. A transfer pattern is formed by the light shielding members 32.
The conventional photomask 30 is manufactured in the following manner, for example. A first step of fabrication is formation of a thin chromium film 33 on the transparent substrate 31 and an E-beam-sensitive resist layer 34 on the thin chromium film 33 (FIG. 10). Next, a predetermined pattern is drawn with an electron beam 35 on the E-beam-sensitive resist layer 34. This is followed by development by which the E-beam-sensitive resist layer 34 is patterned as shown in FIG. 11. Then, the thin chromium film 33 is etched through the E-beam-sensitive resist layer 34, thereby defining the light shielding members 32 as patterned as shown in FIG. 12. At last, the E-beam-sensitive resist layer 34 is removed so that the photomask 30 as that shown in FIG. 13 is obtained.
In a projected image of the photomask 30, as seen in FIG. 14A which shows the amplitude distribution of transmitted light, due to diffraction, light passed through the transparent substrate 31 of FIG. 14A floods into light shielding areas under the light shielding members 32. Since an actual light intensity is a square of the amplitude, the distribution of the light intensity is as shown in FIG. 14C. Thus, in the conventional photomask, light is present in the light shielding areas which are created by the light shielding members 32, which deteriorates the pattern transfer resolution and hence the transfer accuracy of fine patterns.
As a method of preventing such diffraction-induced deterioration in resolution, the phase shift method is well known in the art. The phase shift method requires that a photomask 50 has a transfer pattern in which transparent parts T1, T2 . . . and light shielding parts S1, S2, S3 . . . are alternately arranged and that a phase shifting member 53 is disposed on every other transparent part as shown in FIG. 15. That is, in the transparent part T2, the phase shifting member 53 is formed on the transparent substrate 51 between adjacent light shielding members 52. The phase shifting members 53 each have a thickness which gives rise to 180-degree phase difference between light therethrough and light not therethrough.
Hence, light passed through the transparent parts T1 and T2 and flooding under the light shielding part S2 interfere and cancel each other. A result is an improved resolution.
The photomask 50 of FIG. 15 is manufactured in the following manner, for example. First, similarly to fabrication steps as those shown in FIGS. 10 to 13, the light shielding member 52 having a predetermined pattern is formed on the transparent substrate 51 as shown in FIG. 16. Next, a transparent film 54 made of glass, for instance, is formed on the transparent substrate 51 and the light shielding member 52 as shown in FIG. 17. The transparent film 54 is then covered with a resist layer and subjected to drawing with an electron beam or the like, thereby transferring a pattern onto the resist layer. The pattern is then developed so that a resist pattern 55 is obtained which consists of resist layers each disposed on every other transparent part not covered with the light shielding member 52 (FIG. 18). The transparent film 54 is etched using the resist pattern 55 as a mask, whereby the phase shifting members 53 patterned as shown in FIG. 19 are formed. The resist pattern 55 is removed, completing the photomask 50 as that shown in FIG. 15.
The resist photomask 50 of FIG. 15, however, cannot avoid direct contact of an edge portion of the phase shifting member 53 and the transparent substrate 51 at an edge portion of the transfer pattern, e.g., a region indicated at the reference character E in FIG. 20A. Since light passed through both the transparent substrate 51 and the phase shifting member 53 is 180 degrees out of phase with light passed through only the transparent substrate 51 (FIG. 20B), troublesome zero light intensity occurs in the region indicated at E as shown in FIG. 20C. Hence, when the transfer pattern of the photomask 50 as that shown in FIG. 21A is transferred onto, for example, a positive resist 56 formed on a semiconductor substrate, the resist 56 will not be patterned into the configuration as that shown in FIG. 9B as it should be. Instead, the resist 56 will become as shown in FIG. 21B in which unnecessary residual resist exists where it should not exist (i.e., where the unwanted zero light intensity is created).
A transfer pattern actually used in fabrication of a semiconductor device is a finite pattern and therefore naturally has edges. Hence, the phase shift method inevitably creates residual resist in the edge regions. Only solution to this problem is not to dispose the phase shifting member in the edge regions of the pattern. Thus, the phase shift method does not allow that the phase shifting members are arranged entirely over the transfer pattern, deteriorating the resolution where there is no phase shifting member disposed.